This invention is related to a method and apparatus for detecting and remedying malfunctions in the sensors and multiple control systems needed to control the status and operation of vehicles, and especially to a diagnosis control method for comprehensive control of the operation of vehicles, including correction of detected malfunctions, assuring the vehicle's operability, prevention of exhaust gas degradation, prevention of fuel consumption increases, and the like.
It is most important to diagnose the various functions of a vehicle from a safety standpoint, before it is driven. Much attention has thus been paid heretofore to diagnostic technology for vehicles, and various diagnostic devices and methods have been developed. For example, Japanese Patent Laid-Open No. 263241 of 1988 discloses a method of detecting accidental engine misfires (failure to fire or incomplete combustion), in which an air-fuel ratio detector output pattern and a crank point signal are used to judge whether and in which cylinder a misfire occurs. When a misfire is detected and the cylinder is identified, the supply of the fuel to that cylinder is stopped in order to prevent a degradation of the exhaust gas. Since the manner in which exhaust gas emissions will be affected most by a misfire is known, necessary countermeasures can be taken to deal with the misfire. Since no positive steps are taken to remedy the misfire, however, this process may be characterized as a passive diagnostic control.
In addition to foregoing technique for detecting misfires, other diagnostic methods are also known. These include degradation diagnosis (Japanese Patent Laid-Open No. 91440 of 1990), exhaust gas return flow control system diagnosis (Japanese Patent Laid-Open No. 210058 of 1991), O.sub.2 sensor diagnosis (Japanese Patent Laid-Open No. 165558 of 1987), and secondary air suction system diagnosis (Japanese Patent Laid-Open No. 216011 of 1990). Each of the patents referred to above describes a diagnostic method or device which is in the form of a passive control. Most of the methods are purely diagnostic in nature, and are directed to assuring an accurate detection of the malfunction. While it is true that some of them, like the above described misfire detection technique, can effect minimal control measures based on the diagnostic result (such as stopping the supply of the fuel in order to avoid degradation of the exhaust gas characteristics), they still provide only a passive control of the detected problem, with no further steps being taken to actually correct it. On the other hand, since those diagnostic methods will be subject to government regulation, diagnostic technologies proper to those regulations are all the more important.
When a malfunction of some sort is detected, it is most important to determine exactly which countermeasures should be taken under the circumstances. For example, if a misfire is detected, action can be taken to restore proper firing of the cylinder. If, however, the supply of the fuel is stopped unnecessarily, this measure could needlessly lower the power output of the vehicle. If it is possible to remedy the misfire in a manner that permits continued operation of the vehicle, such a control measure is obviously desirable. Of course, this depends to a great extent on the operating status of the vehicle when the misfire is detected, which may limit the range of subsequent controls that can be used. Thus, the optimized controls should be selected sequentially from among the controls available to correct the malfunction, assure vehicle operability, maintain the exhaust gas characteristics and the current fuel consumption, and the like, by taking the safety and the operating status of the vehicle at that time into account.
Thus far, only examples of cylinder misfiring have been described. However, the range of items susceptible to such diagnosis also includes the catalyst, O.sub.2 sensors, O.sub.2 sensor heaters, evaporative fuel systems, EGR valves, secondary air supply, fuel control systems, and the like. The purpose of the present invention, therefore is to provide a comprehensive vehicle diagnostic control system capable of implementing positive control over operation of the subject vehicle in response to the current engine operation status when a malfunction is detected by the existing diagnostic measures.